The present invention relates to a method and apparatus for sealing paperboard packaging material, a laminate including a layer of metallic foil, an exterior layer of a thermoplastic material, and a layer of paper ("polyfoil"). The thermoplastic layer is utilized as the sealant, and the thermoplastic material of one polyfoil area is fused to the thermoplastic material of a second polyfoil area.
Polyfoils having a metallic foil layer such as aluminum foil, an exterior thermoplastic layer such as polyethylene or polypropylene, and a structural paperboard layer have been used advantageously in making containers for food products and liquid products. Polyfoil is impermeable to liquid, resistant to fatty substances, acids, and the like, and can be sealed to itself by heating and pressing together two facing thermoplastic surfaces so that the thermoplastic material softens and fuses together. Upon removing the heat, the thermoplastic material hardens as one mass, forming a substantially airtight seal, preferably a hermetic seal.
In one commercial practice, a single web of polyfoil paperboard laminate is folded over, heated, and sealed longitudinally to form a tube. The tube is sterilized before, during or after construction, filled with a fluid such as milk or orange juice, and sealed transversely, enveloping the substance in a hermetically sealed, airtight, sterile package. The envelope is then severed from the tube being continuously formed above, through the area seaed, resulting in a discrete package having seals at side, top, and bottom, and a seal at the bottom or end of the tube. The container may then be formed into a parallelepiped or tetrahedal for handling and shipping.
To achieve the seal, the opposing thermoplastic materials must be pressed together, heated, softened, fused, and allowed to cool before the pressure is removed. Several methods of heating the thermoplastic materials have been used. One technique is using radiant heat; heating the thermoplastic by conduction. Typically heater elements are located in the same means that compresses the polyfoil together, for example, resistance welding jaws having Nichrome wire or other resistance element at the pressure surface or heated pressure belts. E.g., U.S. Pat. Nos. 3,940,305 to Stenberg, 3,927,297 and 3,948,720 to Reil, 3,874,976 to McFarland, Jr., 3,140,218 to Hannon, 2,621,704 to Langer and 2,542,901 to Chaffee. Other techniques apply heat radiantly to the plastic, for example, using a quartz lamp and a heat transmitting pressure diaphram as shown in Abramson et al. U.S. Pat. No. 3,472,721. One further technique is to heat the heating element by induction at one location and then move the heated element to a second location where it contacts and heats the thermoplastic material by conduction, e.g., Garbini et al. U.S. Pat. No. 3,883,386.
The use of conduction heating is inefficient for sealing polyfoil. The paperboard layer acts as an insulator and if the heat is too high the paper will ignite before the thermoplastic material has fused. The metallic foil layer acts to dissipate the heat before it reaches the thermoplastic. This results in wasting excessive amounts of energy in heating the paperboard and foil to heat the thermoplastic. Furthermore, some means of cooling the softened thermoplastic while it remains under pressure must often be provided to form an integral seal.
A second technique that has been used with some thermoplastic materials and heat curable adhesives involves heating the material dielectrically; generating an electromagnetic field having a frequency in a range from about 1 MHz to 2.4 GHz, to induce a dielectric current in the material directly. These dielectric currents generate heat that will soften thermoplastic material and activate thermosetting adhesives. Electromagnetic fields for dielectric heating of thermoplastic or thermosetting materials can be generated between electrically conducting endless belts for heating sheets of synthetic resins, e.g., U.S. Pat. Nos. 4,316,709 to Petersson et al., and 2,492,530 to Kriegsheim; or by passing the materials to be heated over openings in waveguides, e.g., U.S. Pat. Nos. 4,060,443 to Balla, 3,109,080 to Pungs et al., and 2,506,626 to Zotto. Microwave energy can also be used to cure heat curable epoxy adhesives, e.g., U.S. Pat. Nos. 4,186,044 to Bradley et al., 4,160,144 to Kayshap et al., 3,707,773 to Wolfberg et al., 3,027,443 to Reed et al., 2,708,649 to Cunningham, 2,631,642 to Richardson et al., 2,612,595 to Warren, 2,479,290 to Auxier et al.
Dielectric heating is not a practicable technique because the thermoplastics used in commercial paperboard polyfoil laminates have low dielectric losses. This makes it difficult to induce a current and requires excessive power to generate strong electromagnetic fields or long exposure periods to adequately heat the thermoplastic layer.
A third technique of heating is by induction; subjecting the compressed polyfoil laminates to an alternating electromagnetic field having a frequency in a range from about 5 KHz to 2 MHz, to induce an eddy current in the area of the metallic foil layers subject to the electromagnetic field. These induced currents generate heat in the foil because of resistance losses and the heat softens the adjacent thermoplastic layers by conduction. Examples of induction heating typically place an induction coil, electrically connected to a high frequency generator, adjacent the area where the current is to be induced. Commercial high frequency generators have powers varying from a few watts to about 30 kw. In some examples, the current is induced in a metallic foil layer adjacent the thermoplastic material e.g., U.S. Pat. Nos. 4,248,653 to Gerger, 3,864,186 to Balla, 3,873,961 to Vouillemin, 3,730,804 and 3,879,247 to Dickey, 3,723,212 to Casper, 3,556,887 to Adcock et al., and 3,424,885 to Garney et al. In other examples the current is induced in other conductive materials, such as finely divided carbon particles or iron oxide applied to or sandwiched between thermoplastic materials, e.g., U.S. Pat. Nos. 4,264,668 to Balla, 3,730,804 and 3,879,247 to Dickey, 3,652,361, 3,462,336 and 3,396,258 to Leatherman, 3,574,031 to Heller, 3,367,808 to Edwards and 3,450,856 to Buck.
The package forming machines using induction heating to form transverse seals presently available cannot be adapted to operate fast enough to be economical and competitive because the electromagnetic field is generated by an induction coil that is electrically and directly connected to the high frequency generator. Furthermore, induction coils that operate continuously, for lengthy periods of time must also have a cooling means for preventing the coil from overheating, melting, and shorting the generator, e.g., a source of circulating water connected to and flowing through a hollow portion of the coil. The electrical and cooling connections restrict the movement of the induction coil. These machines are primarily limited to intermittent sealing, where the coil only moves toward and away from the polyfoil laminate, and slow reciprocation because the distance an induction coil can travel in the direction of travel of the polyfoil is limited by the length of the electrical and cooling connections, and the coil must return to the starting point for each seal. Consequently, any continuous production of sealed packages would be very slow, inefficient, and uneconomical.
Secondary induction coils generate electromagnetic fields by being subjected to a first electromagnetic field generated by a primary coil electrically connected to the generator, having a current induced, and using that induced current to generate the second field. They are not directly connected to a high frequency generator. The problem with known secondary induction systems is that they were designed for soldering, brazing, or heat treating small parts such as capacitors or resistors. These methods use plate conductors that are mounted on turntables or translating frames to move the coil in and out of the primary coil's electromagnetic field. The known methods do not have the rigidness required to simultaneously exert pressure on and inductively heat the workpiece in cooperation with a pressing surface, nor the speed or control necessary to seal commercially available polyfoil materials. Furthermore, these secondary induction coils could never be used to compress the workpiece because of the severe damage that would result, e.g., shorting the generator, burning out the coil.